A strain measurement of an optical fiber using optical frequency domain reflectometry (OFDR) has been hitherto performed.
The basic configuration of an optical frequency domain reflectometer (hereinafter, simply referred to as a measurement device) 200 is shown in FIG. 18. In FIG. 18, a wavelength swept light source 1 includes a semiconductor laser, and outputs wavelength swept light P0 whose frequency changes linearly with respect to time.
Wavelength swept light P0 is input to split means 3 constituted of an optical coupler or the like and is split into two light beams, and one split light beam P1 is guided to one end of a measurement-target optical fiber 38 through directional coupling means 31 constituted of an optical circulator or the like as measurement light Pmes.
Reflected light Pret which is reflected inside the measurement-target optical fiber 38 and returned to one end is input to combine means 41 constituted of an optical coupler or the like through the directional coupling means 31. The other split light beam P2 (=reference light Pr) split by the split means 3 is also input to the combine means 41, and reflected light Pret interferes with reference light Pr.
In two light beams Psum(+) and Psum(−) output from the combine means 41, the phases of the light beams interfering with each other are opposite to each other. The two light beams Psum(+) and Psum(−) output from the combine means 41 are input to a balanced photodetector 55 to detect a beat signal due to interference of reflected light Pret from the measurement-target optical fiber 38 and reference light Pr.
An analog electrical signal A output from the balanced photodetector 55 is converted to a digital signal D by an A/D converter 65, and is subject to Fourier transform processing or the like in a signal processing unit 90.
Here, as shown in FIG. 19(a), three reflection points of a point, b point, and c point are assumed in the measurement-target optical fiber 38, and the distances from an end point o of the measurement-target optical fiber 38 are referred to as La, Lb, and Lc.
If the optical path length from the split means 3 to the combine means 41 while being folded at the end point o of the measurement-target optical fiber 38 is made equal to the optical path length from the split means 3 to the combine means 41 through which reference light Pr propagates, light Preta reflected from the a point of the measurement-target optical fiber 38 is combined by the combine means 41 while being delayed by the time ta=2nLa/c compared to reference light Pr. Here, n is a refractive index of the measurement-target optical fiber 38, and c is the speed of light. Similarly, light Pretb and light Pretc reflected from the b point and the c point are delayed by the time tb=2nLb/c and the time tc=2nLc/c.
Accordingly, as shown in FIG. 19(b), an optical frequency νr of reference light Pr, an optical frequency va of reflected light from the a point, an optical frequency νb of reflected light from the b point, and an optical frequency νc of reflected light from the c point have change characteristics (in this case, lines having a constant inclination) of the optical frequency of reference light Pr in terms of time while being delayed by the time ta to tc.
If an optical frequency variation per unit time of wavelength swept light P0 is S, a beat frequency due to interference of reflected light Preta from the a point and reference light Pr is as follows.fa=|νa−νr|=S·ta=(2nS/c)La  (1)
Similarly, beat frequencies due to interference of reflected light from the b point and the c point and reference light are as follows.fb=|νb−νr|=S·tb=(2nS/c)Lb  (2)fc=|νc−νr|=S·tc=(2nS/c)Lc  (3)
Accordingly, in the signal processing unit 90, if Fourier transform is performed on the digital signal D, as shown in FIG. 19(c), beat signals having frequencies fa, fb, and fc proportional to the distances La, Lb, and Lc from one end of the measurement-target optical fiber 38 to the reflection points are observed. It is assumed that reflectance at each point is sufficiently small, and multiple reflection is ignored.
As described above, the longitudinal distribution of reflection from the measurement-target optical fiber can be measured by the optical frequency domain reflectometry.
If light is continuously reflected in the longitudinal direction due to Rayleigh scattering of the measurement-target optical fiber and longitudinal strain of the measurement-target optical fiber is applied, the phase of reflected light due to Rayleigh scattering changes.
For this reason, the longitudinal distribution of fine strain of the measurement-target optical fiber can be measured by observing the phase of the beat signals obtained by the optical frequency domain reflectometry described above.
Patent Document 1 describes a method which applies the optical frequency domain reflectometry described above to a multicore fiber having a plurality of cores and measures the position or the shape of the fiber.
In the configuration example of FIG. 18 described above, although polarization of light is not considered, in a normal single mode fiber, polarization of light is not maintained and polarization is changed with bending of the fiber; therefore, there is a problem in that, if reflected light from the measurement-target optical fiber is orthogonal to polarization of reference light, beat signals due to interference are not obtained. When the optical fiber is bent, birefringence with different refractive indexes is generated depending on the polarization state of light, and affects a phase measurement by the optical frequency domain reflectometry.
In order to solve the problem, a polarization diversity system is used in which light having two orthogonal polarization states is incident on an optical fiber while switching for each sweep, and each polarization state is separated and measured into two orthogonal polarization components of reflected light from the optical fiber.
FIG. 20 shows a configuration example of a measurement device 210 using a polarization diversity system of the related art. In the measurement device 210, similarly to the measurement device 200 shown in FIG. 18 described above, wavelength swept light P0 output from a wavelength swept light source 1 is split into two beams by split means 3; however, one split light beam P1 is input to a polarization controller 15. The polarization controller 15 switches the polarization state of emission light to a first polarization state and a second polarization state orthogonal to the first polarization state, and the two polarization states are switched by a controller 16 each time a wavelength sweep is performed with the wavelength swept light source 1.
Similarly, output light P1′ of the polarization controller 15 is input to the measurement-target optical fiber 38 through directional coupling means 31 as measurement light Pmes, and reflected light Pret from the measurement-target optical fiber 38 with respect to measurement light Pmes is input to combine means 41 through the directional coupling means 31.
The other light beam P2 split by the split means 3 is input to a polarization controller 25. The polarization controller 25 is adjusted such that the intensities of reference light split into two light beams by polarization separation means 45 and 46 described below become substantially equal. The polarization controller 25 is not required when the polarization state of wavelength swept light P0 is set in advance such that the intensities of reference light split into two light beams by the polarization separation means 45 and 46 become substantially equal.
A light beam P2′ output from the polarization controller 25 is input to the combine means 41 as reference light Pr along with reflected light Pret from the measurement-target optical fiber 38 and combined with reflected light Pr, and reflected light Pret and reference light Pr interfere with each other. As described above, in the two light beams Psum(+) and Psum(−) output from the combine means 41, the phases of light interfering with each other are opposite to each other, one light beam Psum(+) is input to the polarization separation means 45 constituted of a polarization beam splitter (PBS) or the like and split into two orthogonal polarization components s(+) and p(+). The other light beam Psum(−) is input to the polarization separation means 46 constituted of a PBS or the like and split into two orthogonal polarization components s(−) and p(−).
The separated polarization components s(+) and s(−) are input to a balanced photodetector 55, and an electrical signal As proportional to the difference in light intensity of the polarization components s(+) and s(−) is output and converted to a digital signal Ds by an A/D converter 65. Similarly, the separated polarization components p(+) and p(−) are input to a balanced photodetector 56, and an electrical signal Ap proportional to the difference in light intensity of the polarization components p(+) and p(−) is output and converted to a digital signal Dp by an A/D converter 66.
The digital signals Ds and Dp are input to a signal processing unit 91 constituted of a CPU or the like and subjected to Fourier transform processing.
Patent Document 1 discloses a technique which, in the measurement device 210 configured as above, when the Fourier transform results of the digital signals Ds and Dp obtained when a wavelength sweep is performed with the polarization controller 15 set in the first polarization state are respectively a and b, and the Fourier transform results of the digital signals Ds and Dp obtained when a wavelength sweep is performed with the polarization controller 15 set in the second polarization state are respectively c and d, corrects birefringence of the measurement-target optical fiber 38 from the four Fourier transform results a, b, c, and d.
Even when birefringence of the measurement-target optical fiber 38 is not corrected, in a normal single mode fiber, polarization of light is not maintained and polarization is changed with bending of the fiber; therefore, in order to solve a problem in that, if reflected light from the measurement-target optical fiber and polarization of reference light are orthogonal to each other, a beat signal by interference is not obtained, the polarization controller 25, the polarization separation means, and two sets of photodetectors and A/D converters are required, and the polarization controller 25 needs to be adjusted such that the intensities of reference light split into two light beams by the polarization separation means are substantially equal.
FIG. 21 shows a configuration example of another measurement device 220 using a polarization diversity system of the related art. In the measurement device 220, output light P0 from a wavelength swept light source 1 is provided to a polarization controller 15 before being split by split means 3, and the polarization state of output light is switched to a first polarization state and a second polarization state orthogonal to the first polarization state for each wavelength sweep.
Then, reflected light Pret from the measurement-target optical fiber 38 and reference light Pr are combined by combine means 41, and combined light Psum is input to a polarization controller 25. Similarly to the above, the polarization controller 25 is adjusted such that the intensities of reference light split into two light beams by polarization separation means 45 described below become substantially equal.
Output light Psum′ of the polarization controller 25 is separated into light in polarization states s and p by the polarization separation means 45.
In FIG. 21, instead of the balanced photodetectors 55 and 56 of FIG. 20, single-end photodetectors 57 and 58 are used. A digital signal Ds is obtained from light in the polarization state s, a digital signal Dp is obtained from light in the polarization state p, and as described referring to FIG. 20, birefringence of the measurement-target optical fiber 38 can be corrected by the method described in Patent Document 1 from Fourier transform results a and b obtained when a wavelength sweep is performed with the polarization state of output light of the polarization controller 15 set in the first polarization state and Fourier transform results c and d obtained when a wavelength sweep is performed with the polarization state of output light of the polarization controller 15 set in the second polarization state.
FIG. 22 shows the configuration of a measurement device 230 which measures a three-dimensional position or shape using a multicore fiber 39 with the configuration shown in FIG. 21 as a basic configuration.
In the measurement device 230, output light P0 from the wavelength swept light source 1 is split by split means 2, similarly to the above, one split light beam P1 is input to a polarization controller 15, and the other split light beam P2 is input to a monitoring unit 70.
As shown in FIG. 23, the monitoring unit 70 splits input light P2 into two light beams by split means 71, provides one light beam to a gas cell 72 of hydrogen cyanide (HCN), measures power of light passing through the gas cell 72 by a photodetector 73 and an A/D converter 74, and outputs the measurement result to a signal processing unit 92. The signal processing unit 92 calibrates the absolute wavelength of the wavelength swept light source 1 according to the absorption wavelength of gas.
The other light beam split by split means 71 is provided to a delay interferometer constituted of an optical coupler 81, a delay fiber 82, and Faraday rotator type mirrors 83 and 84. The output of the delay interferometer is measured by a photodetector 85 and an A/D converter 86. A sine wave of a beat frequency according to a change in the optical frequency of the wavelength swept light source 1 is obtained from the output of the delay interferometer. In the actual wavelength swept light source 1, the change in the optical frequency with respect to time is not completely linear; therefore, the signal processing unit 92 performs correction processing on nonlinearity of a wavelength sweep using the output of the delay interferometer described above.
On the other hand, similarly to the above, light P1′ whose polarization state is switched for each wavelength sweep is output from the polarization controller 15 and split into four light beams by split means 30. The four split light beams P3 to P6 are respectively input to split means 3A to 3D and split into measurement light Pmes1 to Pmes4 and reference light Pr1 to Pr4, and the four measurement light Pmes1 to Pmes4 are input to a fan-out 35 for a multicore fiber respectively through directional coupling means 31A to 31D and input to respective cores of one measurement-target multicore fiber 39.
Reflected light Pret1 to Pret4 from the respective cores of the measurement-target multicore fiber 39 are respectively input to combine means 41A to 41D through the fan-out 35 for a multicore fiber and the directional coupling means 31A to 31D.
Reference light Pr1 to Pr4 are also respectively input to the combine means 41A to 41D, and similarly to the configuration of FIG. 21, reflected light from the respective cores of the measurement-target multicore fiber 39 and reference light are combined and respectively separated into two polarization components (s1,p1) to (s4,p4) by polarization separation means 45A to 45D upon receiving the output. The two polarization components (s1,p1) to (s4,p4) are converted to electrical signals by photodetectors 57A to 57D and 58A to 58D, the electrical signals are converted to digital signals (Ds1,Dp1) to (Ds4,Dp4) by A/D converters 65A to 65D and 66A to 66D, and the digital signals (Ds1,Dp1) to (Ds4,Dp4) are input to the signal processing unit 92. Similarly to the above, polarization controllers 25A to 25D are adjusted such that the intensities of reference light split into two light beams by the subsequent polarization separation means 45A to 45D become substantially equal.
With this configuration, similarly to FIG. 21, birefringence can be corrected and the strain distributions of the respective cores of the measurement-target multicore fiber 39 can be measured. In addition, the position or the shape of the measurement-target multicore fiber 39 can be calculated from the strain distributions of the respective cores.
Patent Document 2 describes a method which uses a fiber Bragg grating (FBG) as a measurement-target optical fiber. The reflectance of the FBG is higher than the reflectance of Rayleigh scattering; therefore, it is possible to reduce the influence of reflection in the termination of the measurement-target optical fiber, an optical connector, the directional coupling means (optical circulator), or the like and crosstalk of the measurement-target multicore fiber or the fan-out. In addition, with the use of the FBG with a chirped reflection wavelength, reflected light is obtained over a wide wavelength sweep range, and the dynamic range of the photodetector can be suppressed.